1. Field of the Invention
The present invention relates generally to an optical deflector for deflecting a light beam, and more particularly to techniques for improved deflecting speed and scanning resolution, and for increased angular range of deflection of the light beam.
2. Discussion of the Prior Art
An optical deflector device utilizing a thermo-optic effect is known as a totally solid optical deflector which uses no mechanical moving parts. This totally solid optical deflector is higher in durability and smaller in size, than optical deflectors which include moving parts, for example, a polygon mirror or a hologram scanner. Further, the totally solid optical deflector device does not require complicated and expensive elements such as a frequency modulator capable of high-frequency modulation, or a ultrasonic transducer, as required in an optical deflector which operates utilizing an acousto-optical effect. An example of the totally solid optical deflector is shown in FIG. 3, wherein an electrically resistive heat-generating layer 52 is formed on a surface of a substrate 50 which has a thermo-optic effect. When the heat-generating layer 50 is energized with an electric power supplied from a power source 54, there is produced a temperature gradient in the substrate 50, in the direction of thickness. While no temperature gradient exists in the substrate 50, a light beam incident upon the substrate 50 propagates straight forward through the substrate 50, and is emitted as indicated at 58. However, when a temperature gradient is present in the substrate 50 in the direction of thickness, the incident light beam 56 is deflected in the direction of thickness of the substrate 50, at an angle corresponding to the temperature gradient formed in the substrate. The scanning resolution N of the deflector, i.e., the number of beam spots which form a scanning line, is generally expressed by the following Equation (1): EQU N=(2.pi./.lambda.).multidot..theta.D (1)
where,
.theta.: angle of deflection of light beam PA1 .lambda.: wavelength of light beam PA1 D: cross sectional size of light beam, measured at the output end of the deflector, in the direction of deflection.
In the conventional optical deflector as shown in FIG. 3, the deflection angle .theta. is generally limited. To improve the resolution N, therefore, the size D of the light beam must be increased. This means that the thickness of the substrate must be increased to improve the resolution N. An increase in the substrate thickness results in an increase in the heat capacity of the substrate, which leads to an increased time required for forming a desired temperature gradient in the substrate. Thus, an increase in the substrate thickness for improvement in the resolution N will deteriorate the light deflecting speed of the optical deflector, which is one of the basic capabilities required on an optical deflector.
Also known is a totally solid optical deflector wherein the refractive index of an optical waveguide formed in the substrate is changed by utilizing an electro-optical, acousto-optical or thermo-optic effect. This type of optical deflector as presently available, which uses no mechanical moving parts, is also compact and sufficiently durable, but is still unsatisfactory in its capability of deflecting a light beam over a sufficiently wide angular range.